f2fs: report the discard cmd errors properly

In case a discard_cmd is split into several bios, the dc->error
must not be overwritten once an error is reported by a bio. Also,
move it under dc->lock.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>